HORTICULTURAL ENTOMOLOGY Impact of Insecticide Efficacy on Developing Action Thresholds for Pest Management: A Case Study of Onion Thrips (Thysanoptera: Thripidae) on Onion

An action threshold (AT) is one of the most important decision-making elements in integrated pest management. Unlike economic thresholds, ATs are not typically derived from an economic injury level model, but they are more commonly used. ATs may be identiÞed from research-based, pestÐcrop relationships, but they also may be based on experience. ATs may be adjusteddependingon, e.g.,weather andplant variety, butmodifyingATs toaccommodatedifferences in insecticide efÞcacy has received little attention. To examine this point, several combinations of ATs and insecticides were evaluated against onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), a major pest of onion (Allium cepa L.). Studies were conducted in New York onion Þelds from 2006 to 2008 by using registered insecticides for T. tabaci on onions. We hypothesized that the most efÞcacious insecticides would provide acceptable control of thrips populations regardless of AT (one, three, and Þve thrips per leaf), whereas less effective products would only control populations using the lowest AT (one thrips per leaf). Results indicated that T. tabaci infestations were managed effectively when spinetoram was applied after a three larvae per leaf threshold, but not when using lambda-cyhalothrin, methomyl or formetanate hydrochloride. However, T. tabaci infestations were managed well whenmethomyl and formetanate hydrochloride were applied after a one larva per leaf threshold. T. tabaci infestations were never controlled using lambda-cyhalothrin, regardless of the AT used. None of the products reduced T. tabaci populations to an acceptable level when applied at a Þve larvae per leaf threshold. Implications of adjusting ATs based on efÞcacy of different insecticides are discussed.